1. Field of the Invention
This invention relates in general to data storage systems, and more particularly to a method and apparatus for detecting and computing tape reel radius using low-resolution angular position sensors.
2. Description of Related Art
Data loss is a serious threat to companies of all sizes, and catastrophic data loss can destroy a business. But most data loss isn""t the result of a big disaster, it""s caused by human error, viruses, and disk malfunctions. A suitable backup routine provides the best protection against data loss of all kinds. And tape technology remains the most efficient and cost-effective means to perform system backup, whether for a small business or a global 24xc3x977 operation.
Tape remains unrivaled in terms of cost and capacity for data storage, and should play an increasing crucial role in corporate data protection strategies. No other technology offers the same combined low cost and high capacity advantage of tape. While other technologies may offer strengths in one or more areas, overall, they do not meet the entire set of customer needs that tape addresses.
Tape drives make backup fast, easy, reliable and affordable. Speed is critical because your data is constantly growing while the time available for backup is shrinking. Even the slowest tape drive writes 1 MB per second and the fastest 30 MB per second, which means a 200 GB backup can be completed in less than two hours. Furthermore, unlike other storage methods, tape drives offer a range of media that allows you to back up all the data on a small to medium-sized server. Tape backup also captures system setup information, as well as data, allowing an entire system to be restored in the event a disaster strikes. Also, backups can be scheduled to occur automatically at a time determined to be most convenient.
Another area where tape storage excels is when it comes to data protection. Tape has proved itself a reliable medium, and tape drives themselves have never been more reliable. Easily portable, tapes have the added advantage of being simple to remove and store offsite, so keeping a disaster recovery copy is less of a burden. In terms of affordability, tape is the most cost-effective way to store large amounts of data per gigabyte of storage. The compact size of tape cartridges also helps keep down your storage costs.
Tape motion control in a tape drive, where the tape directly couples a take-up reel and supply reel, requires that the radius of the tape wound on each reel be determined. This is necessary to control tension, position, velocity, and acceleration of the tape when large variation in reel radius occur as the tape is moved from one reel onto the other. The current techniques for determining radius require at least one fine-resolution tachometer and one low resolution tachometer.
Prior techniques, which have been employed in tape drives to determine reel radius of the tape reels in a tape drive, rely on counting the number of fine-line pulses of a digital tachometer connected to one reel that occur during one revolution of the other reel, as signaled by a coarse tachometer, such as one pulse per turn. This count is proportional to the ratio of the reel radii, R1/R2 by the following relationship:
N=K(R2/R1).
where, N=Count of fine line pulses, K=Number of pulses per turn of the fine-line tachometer, R1=Radius of the reel connected to the fine tachometer, and R2=Radius of the reel connected to the one pulse per turn tachometer. This equation, which has two unknowns, R1 and R2, can be solved by employing one other equation derived from the sum of the area of tape wound on the reels. That equation is given by the relationship:
R12+R22=LT/pi+2R02
where L=Total length of tape and T=Thickness of tape and R0 is the empty reel hub radius. These two equations are employed to solve for R1 and R2 uniquely as:
R1=((LT/pi+2R02)/(1+N2/K2))0.5 and
R2=((LT/pi+2R02)/(1+K2/N2))0.5.
However, this technique requires at least one fine line tachometer in order to provide an acceptable measurement resolution for radius which can be updated every revolution of the coarse line tachometer.
Another prior technique employed in the IBM 8809 tape drive uses a single fine line tachometer mounted on a roller in the tape path and driven by the tape. Each reel has a single pulse per turn index pulse tachometer. The number of fine line tachometer pulses are counter for each revolution of the reels. The resulting counts are directly proportional to reel radius.
Additionally, the evolving Linear Tape Open (LTO) technology provides a servo track pre-recorded on the tape which is composed of a repeated regular pattern of flux transitions. The new LTO tape product uses a tape format that has longitudinally pre-written servo tracks. The servo tracks provide a timing-based track-following position error system. The tracks contain a repeated pattern of recorded flux transitions that occur as grouped bursts of 5, 5, 4, and 4 transitions. The timing between the sets of 5-bursts and between sets of 4-bursts provides the position information for the track following system. Additionally, the individual transitions within the 5-bursts are phase-shifted in a manner that encodes longitudinal position information (LPOS) into the servo tracks.
By detecting the phase-encoded LPOS information, the tape transport system determines the tape position relative to landmarks lengthwise down the tape. The LPOS information is used to keep track of the longitudinal position of data records written onto or read from the tape, and is used to locate those data records when the reading or writing process temporarily stops. The LPOS location of data files on tape is also stored in the volume control data for use to locate the data files during a later tape cartridge load for reading, or for write-appending new files onto the end of the last file written to the tape. The LPOS data is thus used as the primary positional information for the tape transport servo control system, it is used in the decision process for starting and stopping the tape, and for backhitching the tape in order to position the read-write heads at the beginning of a data record at the required velocity and track position which allows the start of a new data transfer operation.
Moreover, this servo track can be used to compute the radius of the two reels. However there can be long lengths of time when the servo track is not available. During such times, reel radius must somehow be detected from sensors of mechanical motion of the tape transport hardware.
It can be seen that there is a need for a method and apparatus for detecting and computing tape reel radius using low-resolution angular position sensors.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method and apparatus for detecting and computing tape reel radius using low-resolution angular position sensors.
The present invention solves the above-described problems by calculating the radius of the two reels using two low resolution (one pulse per turn) sensors. This results in a great cost and product packaging savings.
A method in accordance with the principles of the present invention includes driving tape reels at a constant velocity, detecting a complete rotation of each of the tape reels using coarse commutation sensors and calculating a radius for a first reel, R1, and a radius for a second reel, R2 according to:
R1=((L*T/pi)/(1+T22/T12))0.5 and
R2=((L*T/pi)/(1+T12/T22))0.5,
where L is the total length of tape, T is the thickness of tape, T1 is the rotation time for reel 1, and T2 is the rotation time for reel 2.
Other embodiments of a method in accordance with the principles of the invention may include alternative or optional additional aspects. One such aspect of the present invention is that the coarse commutation sensors comprise Hall effect sensors.
In another embodiment of the present invention, a method of detecting and computing tape reel radius is disclosed. The method includes computing tape reel radii using servo track burst of flux transitions when servo tracking is available as a basis for radii computations and computing tape reel radii using a secondary velocity source when servo tracking not is available as a basis for radii computations.
Another aspect of the present invention is that the computing tape reel radii using a secondary velocity source further includes driving tape reels at a constant velocity, detecting a complete rotation of each of the tape reels using coarse commutation sensors and calculating a radius for a first reel, R1, and a radius for a second reel, R2, according to:
R1=((L*T/pi)/(1+T22/T12))0.5 and
R2=((L*T/pi)/(1+T12/T22))0.5,
where L is the total length of tape, T is the thickness of tape, T1 is the rotation time for reel 1, and T2 is the rotation time for reel 2.
Another aspect of the present invention is that the coarse commutation sensors comprise Hall effect sensors.
In another embodiment of the present invention, a tape drive configured for detecting and computing tape reel radii is disclosed. The tape drive includes a first and second tape reel motor, a commutation sensor, coupled to the each of the first and second tape reel motors, for detecting a complete revolution of a first and second tape reel and for providing an index signal in response thereto and a controller, coupled to the first and second tape reel motors, the controller driving the tape reel motors at a constant velocity and receiving the index signal for each of the first and second tape reels, wherein the controller computes a first tape reel radius, R1 and a second tape reel radius, R2, according to:
R1=((L*T/pi)/(1+T22/T12))0.5 and
R2=((L*T/pi)/(1+T12/T22))0.5,
where L is the total length of tape, T is the thickness of tape, T1 is the rotation time for reel 1, and T2 is the rotation time for reel 2.
Another aspect of the present invention is that the coarse commutation sensors comprise Hall effect sensors.
Another aspect of the present invention is that the tape drive further includes a motor driver disposed between each of the first and second tape reel motors and the controller.
In another embodiment of the present invention, an article of manufacture comprising a program storage medium readable by a computer is disclosed. The medium tangibly embodies one or more programs of instructions executable by the computer to perform a method for detecting and computing tape reel radii using low resolution angular position sensors, wherein the method includes driving tape reels at a constant velocity, detecting a complete rotation of each of the tape reels using coarse commutation sensors and calculating a radius for a first reel, R1, and a radius for a second reel, R2, according to:
R1=((L*T/pi)/(1+T22/T12))0.5 and
R2=((L*T/pi)/(1+T12/T22))0.5,
where L is the total length of tape, T is the thickness of tape, T1 is the rotation time for reel 1, and T2 is the rotation time for reel 2.
In another embodiment of the present invention, an article of manufacture comprising a program storage medium readable by a computer is disclosed. The medium tangibly embodies one or more programs of instructions executable by the computer to perform a method for detecting and computing tape reel radius, wherein the method includes computing tape reel radii using servo track burst of flux transitions when servo tracking is available as a basis for radii computations and computing tape reel radii using a secondary velocity source when servo tracking not is available as a basis for radii computations.
Another aspect of the present invention is that the computing tape reel radii using a secondary velocity source further includes driving tape reels at a constant velocity, detecting a complete rotation of each of the tape reels using coarse commutation sensors and calculating a radius for a first reel, R1, and a radius for a second reel, R2, according to:
R1=((L*T/pi)/(1+T22/T12))0.5 and
R2=((L*T/pi)/(1+T12/T22))0.5,
where L is the total length of tape, T is the thickness of tape, T1 is the rotation time for reel 1, and T2 is the rotation time for reel 2.
These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance with the invention.